The steel and foundry industries worldwide are quite large in numbers and production. A multitude of raw materials are used in these facilities. These industries like others are interested in saving money and keeping a better public profile. This is partially achieved by recycling waste and by products. We will look at one material or by product that was once known as a nuisance for steel mills and foundries, Mill Scale.
What is Mill Scale and How is it Recycled?
Mill scale can best be described as a grey or black flaky material that forms of steel and iron. This material forms on the exterior surfaces of plates, longs, billets and sheets while they’re in the initial manufacturing process. Mill scale also forms when steel is re-heated in rolling mills, the material is then collected. Mill scale is piled up and saved as it is a valuable material.
The Composition of Mill Scale
Mill scale consists mainly of magnetite, specifically Fe304, and carries the blue-gray color that is characteristic of steel it formed on. There is also a very thin outer film made of hematite, Fe203, which cannot be seen with the naked eye. This material is formed due to oxidation of steel during operations like hot rolling, hot press, hot forming, heating/reheating, continuous casting and flame scarfing, conditioning, etc. It is basically a rust formed on the surface of the steel/iron.
Mill Scale Uses
Mill scale has many industrial applications such as:
- The two main uses are steel making and the making of cement clinkers.
- Also Used in counterweights like those found in washing machines.
- Fertilizers and fillers
- Iron oxides and salts
- Friction products
- Colored Glass
- Heavy concrete
- Melting charge
- Mineral wool
- Radiation shielding
- Road construction
- Sinter, Recycling material
- Slag formers
- Stone production
- Tool steel
There are several other uses in manufacturing that shows how useful and valuable mill scale is.
Other Mill scale facts.
Mill scale is not considered hazardous and is generally easy to transport. The material pricing is based on the steel and scrap markets, the price changes frequently.
Production takes place at major steel mills and smaller re-heating mills, the materials are collected and sold to “mill services” or other independent buyers. The mill scale is then screened if need by, loaded onto trucks, rail cars or even barges/ships for delivery to waiting customers.
The reuse of mill scale is for one of it’s several valuable properties. The iron units or Fe content has mill scale put back into the iron making cycle. The weight of mill scale makes it useful for density or heavy weighted materials. The Fe203 is very useful for pigments, oxides and iron salts. These factors make mill scale one of the steel industries biggest success stories in recycling. Millions of tons are kept out of landfills and put back into needed products which is a win for all of us.